Hybrid vehicles typically include a high voltage battery pack adapted to provide power to various components of the vehicles, including motors, transmissions, and electrical accessories. A battery pack may consist of various chemistries, such as lithium ion, nickel metal hydride, or nickel cadmium. The battery pack may include multiple cells connected in series, parallel, or series/parallel configurations. Each cell may include a cathode, an anode, and an electrolyte as well as external electrical connections to the anode and cathode. The electrolyte is a medium that allows the ions to flow between the anode and the cathode. The anode and cathode are in electrical contact with metal current collectors which provide a means to deliver and receive electron current to an external device. It is the electrochemical reactions that occur within the cell that allows a battery pack to serve as an energy source.
The performance of the battery packs may degrade overtime because of irreversible chemical reactions at the anode or cathode during normal operation, or during abuse conditions, including but not limited to overcharging, internal shorting, external shorting, and over-discharge, among others. Gas build-up and subsequent increase in pressure within a cell may occur from the decomposition of electrolyte at the surface of the anode or cathode, such as during abuse conditions. The increase in pressure may occur gradually due to slow decomposition of the electrolyte over the cell's life, or may occur rapidly in response to a temperature increase in the cell.